Tom Thumb lettuce plants are a unique variety of butterhead or bibb lettuce that produces crisp buttery leaves which form a loose head. The most unique characteristic of these plants is their diminutive size, growing up to 4-5 inches. This variety is perfect for a wide range of garden applications, including its use as a ?single serving? salad. he process of growing Tom Thumb lettuce is very similar to growing other varieties of lettuce. First, growers will need to determine when it is best to plant the seeds. Since lettuce plants flourish when grown in cooler temperatures, planting most often occurs early in the spring and into the fall in successions.